The present invention relates to generally a process for fabricating solid-state color-image sensors and more particularly a step for bonding to a CCD or BBD color-image sensor chip; that is, a chip bearing a CCD or BBD color-image sensor a color filter having a stripe or mosaic pattern in such a way that the picture-element grid on the chip may be correctly aligned with the stripe or mosaic pattern on the color filter.
Extensive developments of solid-state color-image sensors have raised great expectations in making video cameras compact in size and light in weight. In the case of color video cameras of the type using a single-plate color-image sensor, the color-image sensor must be provided with a three-color filter (for consisting of red, green and cyan blue stripes or mosaic elements) which must be correctly aligned with a picture-element grid on a chip. In general, the color filters are fabricated separately from the color-image chips and comprise an optical glass substrate and a colored pattern layer or a filter pattern which is formed on one major surface of the substrate by coating or depositing colored organic compounds. The color filter is bonded to the color-image sensor chip with a suitable adhesive. A higher degree of adhesive strength may be ensured with the use of an adhesive which is curable by heating and is widely used in cementing optical lenses.
Meanwhile, in order to make the color video cameras more compact in size and lighter in weight, the surface areas of solid-state color-image sensors must be reduced without sacrificing the resolution and the sensitivity to light. For this purpose, spacing between picture elements must be reduced and the colored pattern layer or filter pattern on a color filter must be aligned with the picture-element grid on a chip with a higher degree of accuracy. However, the results of extensive studies and experiments conducted by the inventors that with the conventional thermosetting adhesives, the color filters cannot be bonded to the color-image sensor chips with a desired degree of accuracy in alignment.
For instance, a color filter and a color-image sensor chip each having a bonding surface of 200 mm.sup.2 must be bonded together with an adhesive layer less than 10 .mu.m in thickness and with an alignment accuracy of less than 2 .mu.m, the alignment accuracy referring to an accuracy in alignment between individual picture elements on the chip with their corresponding color filter elements on the color filter. For this purpose, it is imperative to simultaneously carry out the step for pressing the color filter and the color-image sensor chip against each other and the step for attaining the correct alignment between them.
However, with the conventional thermosetting adhesives, curing or setting proceeds very gradually when the adhesive layer is less than 10 .mu.m. As a result, the color filter and the color-image sensor chip cannot be aligned smoothly. Furthermore, when a curing time is considerably longer than an alignment time; that is, a time interval required for attaining the alignment between the color filter and the chip, a long time passes before the chip with the cemented color filter can be removed out of a curing and aligning device. As a result, the mass production of solid-state color-image sensors cannot attain its inherited advantages. Furthermore, there has been a drawback that during a curing time after the alignment step, misalignments tend to occur very frequently. As a result, the signal-to-noise ratio is degraded and color flicker occurs. In the worst case, color misregistration results.
In view of the above, an adhesive used to bond color filters to color-image sensor chips must have such a characteristic that it will not be cured during the alignment step, but will be quickly cured after the alignment. Therefore, it is preferable to use an adhesive of the type which is cured when irradiated by light rays and especially by ultraviolet rays because a cured adhesive layer has a higher degree of transparency. However, it had been found out that adhesives which are curable by ultraviolet-ray irradiation are not satisfactory in practice as will be described in detail with reference to the accompanying drawings.